Unexpected Full Backup -- Should be incremental backup

Win 7-64 SSR 2013 11.0.2.49853 up to date.

I have the incremental backup set to start a new set (? I may have use the wrong term) full backup YEARLY.   However, from time-to-time for some unknowned to me the backup will start an new set. This new unexpected full backup uses up my backup drives disk space and the backup fails.

See the attached file for a list of backups.

Can someone explane  why SSR starts a new set?